Corneal topographers are currently used to obtain curvature and height data and generate topographic maps of a corneal surface of an eye. Compared to keratometers, corneal topographers enable to provide a more detailed description of the corneal surface, e.g. including local variations over the corneal surface, and whereby data of the central and peripheral parts of the cornea can be obtained.
At present, method have been developed to process the height data as obtained from measurements using a corneal topographer to obtain so-called simK indices, such indices being used to characterize astigmatism. Such simK indices e.g. being:                Rmin (or Ksteep)        Rmax (or Kflat)        Angle of astigmatism        
The indices are referred as simK (simulated keratotomy) because the processing of the height data to obtain the astigmatism parameters simulates the operation of a keratometer, which is typically used to characterize astigmatism parameters.
In the known methods to determine the astigmatism parameters, 1-dimensional fits are made along various meridians to determine flattest meridian. This meridian may then be used to determine Kflat, the corresponding angle may be selected as the angle of astigmatism.
For the mentioned method reference can e.g. be made to US 2012/0109595.
A drawback of the known methods to derive global parameters such as astigmatism parameters is that the known methods suffer from a poor repeatability. As a result, when such parameters have been derived, e.g. from 2D corneal height data, an uncertainty may arise whether the derived parameters are sufficiently accurate. In particular, with respect to the angle of astigmatism, a high repeatability is desired because an error of only 10 degrees in the angle of astigmatism would render an astigmatic correction useless.